1. Field of the Invention
The field of this invention relates to application of fluids in industrial fabrication processes, and more particularly to techniques for detecting the presence of contaminants carried in a fluid that is applied to semiconductor wafers during integrated circuit fabrication.
2. Background of the Technical Art
FIG. 1 shows a prior art approach now in use. A spin-on coating (SOC) machine 10 applies a fluid, here a liquid, to a semiconductor wafer undergoing processing. The particular coating machine described here is selected from among the Photoresist Coaters 8300 Series that are manufactured and sold by Silicon Valley Group, Inc. of San Jose, Calif., USA.
The SOC machine 10 includes a process liquid bottle 12, connected as a source to supply liquid through a first tube 14, a pump 16 and a second tube 18 to a local process worksite 20. The bottle 12 is usually sealed inside an airtight chamber 22 within a housing 24 to isolate a process liquid 26a inside the bottle 12 from the ambient environment, typically air. For this discussion, the process liquid 26a, 26b, 26c is present at particular locations in the pump 16 and first and second tubes 14 and 18. Air or other contaminants can mix with process liquids, such as certain liquid solvent-photoresist combinations and particular liquid dielectrics, and form contaminants that degrade the suitability of the liquid for wafer processing.
The pump 16 responds to instructions received from a controller 28 to pump liquid 26a from the bottle 12 as required for the SOC process being performed. The particular liquid volume of a dose will be variable and dependent on such details as the type of process liquid being applied, the thickness of a layer to be formed on a workpiece at the worksite 20, and the surface area of the workpiece that is to be covered with the liquid. The pump 16 moves a dose of liquid 26b through a first tube 14, through the pump 16, and through a second tube 18 for dispensing the liquid through a tube outlet 30 placed adjacent to the worksite 20. A tube outlet 30, sometimes referred to as a nozzle, is positioned at the end of the second tube 18 to control liquid outflow from this tube. Tubes 14 and 18 are typically made with an opaque or mildly translucent material that is flexible so the tubes can bend during semiconductor processing.
At the worksite 20, a turntable 32 is mounted to spin inside a chamber 34 that is defined by a cylinder 36 having a vertically extending splash wall 38. A motor 40 is connected to rotate the turntable 32. A vacuum pump 42 is also coupled to the turntable 32 to generate a sub-ambient pressure at the top surface of the turntable. As a result, the turntable 32 functions as a vacuum chuck, which pulls the backside of an adjacent workpiece 44 against the top surface of turntable 32 so that the workpiece and turntable spin together. Typically the turntable 32 and workpiece 44 are both circular.
During operation of a SOC process, the workpiece 44 is placed onto turntable 32. The vacuum pump 42 is activated to evacuate air from the space between the workpiece 44 and turntable 32, to thereby clamp the workpiece to the turntable. The motor 40 spins the turntable 32, accelerating the turntable and the workpiece 44 to a desired rotational speed ranging broadly from about 1000 rpm up to 8000 rpm, depending on the particular process performed and the number of rotational steps involved in that process.
While the workpiece 44 spins, liquid 26c emerging through tube outlet 30 is received at about the center of the workpiece, as indicated by the liquid 26c. Centrifugal forces immediately disperse the liquid 26c in a circular pattern, causing the liquid to expand radially toward the circumference of the workpiece 44. The liquid 26c quickly forms a new layer 46 having approximately uniform thickness that overlies the top of workpiece 44.
To minimize fouling and contamination of the turntable 32 and the workpiece 44 by excess process liquid, the worksite 20 is designed so that excess liquid is thrown beyond the circumference of the spinning workpiece 44. This excess liquid hits the splash wall 38 and flows down to the bottom surface of the chamber 34.
In another embodiment that minimizes contamination by excess liquid, the diameter of the turntable 32 is made smaller than the diameter of the workpiece 44. However, in the current art, excess liquid cannot flow down the circumference of the workpiece 44 and onto the turntable 32, which is an advantage.
Despite the usefulness of prior art coating devices, certain drawbacks and deficiencies are known to exist with these devices. In particular, a contamination problem persists. Contaminants carried by a liquid aggregate in the coating layer 46 on the workpiece 44 can often degrade the characteristics of the finished workpiece to the point that the workpiece must be reworked or even scrapped. Both remedies are expensive and time-consuming.
Contaminants 48, shown as 48a, 48b, 48c in FIG. 1, are typically present throughout the entire flowpath of the liquid 26, beginning with contaminants 48a within the liquid 26a inside the bottle 12, continuing through the flowing liquid, and forming part of the coating layer 46 on the workpiece 44. The prior art relies on visual inspection of liquid, such as 26c, and of the workpiece 44 at ambient light intensity to find entrained contaminants 48. However, ambient light does not provide sufficient illumination for the contaminants 48 to be easily seen, and many are missed. Further, visual inspection is done relatively late in the SOC process, generally after a contaminant is already incorporated into the workpiece.
What is needed is an approach for quickly and reliably detecting the presence of contaminants carried in a flowing liquid to minimize damage done to a workpiece to which the liquid is applied.